Such a rotor is, for example, known from DE 692 06 626 T2.
With such a rotor, a yoke is formed radially inside the slots, said yoke serving as path for the magnetic flux. Radially inside, the yoke is limited by a bore serving as accommodation for the rotor shaft. If a rotor shaft with a small diameter is used, the bore can accordingly be made smaller. In this case, a relatively large yoke is available, which is not required for magnetic reasons.
Theoretically, the space thus being available, could then be used to extend the slots radially inwards, thus providing a larger line cross-section for the electrical current. The extension of the slots radially inwards, however, has the disadvantage that the slots will be heavily narrowed at the radial inner end. In this case, it will practically not be possible, at least with a defensible effort, to fill the slots completely with an electrically conducting material. Air cavities and other interferences could occur, which could again have a negative effect on the operation behaviour of the machine and reduce the performance.
The problem will be discussed on the basis of FIGS. 7 and 8: If, as shown in FIG. 7, the slots in the rotor are extended radially inwards, this results in a very narrow slot bottom, which can no longer be reasonably filled with electrically conductive material. The limitation of the individual slot in the circumferential direction is pre-specified in that a rotor tooth remaining between two slots must have a certain width, in order to provide a sufficient flow path for the magnetic flux. If, as shown in FIG. 8, these limitations in the circumferential direction would be further extended inwards, a slot would occur, which would geometrically consist of two separate bodies. Logically, this is not a suitable solution.